The answer
Because it sounds like 'undefined,' but they are different. undefined is a value for a declared-but-empty variable. 'not defined' is an error meaning the variable does not exist at all.
ReferenceError: x is not defined in JavaScript
Why is the error message 'not defined' confusing in JavaScript?
More FAQs in ReferenceError: x is not defined in JavaScript
It means the variable x does not exist in any accessible scope. It was never declared, is misspelled, is out of scope, or was not imported. It is different from undefined, which means the variable exists but has no value.
Use typeof. typeof an undeclared variable returns 'undefined' without throwing. For example: if (typeof someGlobal !== 'undefined') { ... } is safe for feature detection.
Accessing a let or const variable before its declaration line throws ReferenceError: Cannot access 'x' before initialization. The message is different from 'not defined' but it is still a ReferenceError.
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